Hi Preacher edski . . .
I love this post - I am a big fan of pondering things from a practical standpoint.
I agree with your analysis of the various circuits in a carb - but I am not sure I follow the conclusions.
First, depending on carb design - most driving is done on the idle circuit or a combination of the idle circuit and the main meter.
I somewhat agree with you about the power circuit - but only if we're talking about a vacuum driven power valve (diaphragm) like in a Holley 94. Not so in a mechanical carb like a Stromberg 97.
Same with a carb equipped with an accelerator pump - it pumps only when your foot transitions the throttle - not when a throttle position is held.
You conclusion about the effect of RPM is the exact opposite of mine. Your analysis of circuits is a discussion of cause - let's focus instead on effect at cruise.
I did a little math on Sunday morning over coffee to demonstrate my reasoning. As you point out in your post - fuel air ratio is critical to both power and to efficiency.
Let's look at the numbers on efficiency first. I find it useful to expand fuel air ratio into something conceptually meaningful to ones every day senses.
Some weights and measures:
Weight . .pounds/gal . .pounds/oz . . .ounces/oz
Gasoline . . . 8.66 . ..0.06765625 . . . .1.0825
Air . . . ounces/cu ft ..cu-in/cu-ft
. . . . . . . 1.2 . . . . . ..1728
Fuel air ratios are calculated by weight:
. . . . . . Air (ounces) .Gas (ounces)
14 to 1 . . . . 15.155 . . . . . 1.0825
12 to 1 . . . . 12.990 . . . . . 1.0825
10 to 1 . . . . 10.825 . . . . . 1.0825
But they are consumed into an engine by volume:
Weight in Ounces . . . . Air volume..| .Total ..|...Air. . . Vapor* . .Fuel consumed
|Air| . . |Fuel| . Ratio...(cu/ft).....|.(cu-in)..|.(cu/in)...(cu-in) . . . . . . (oz)
14.00 . . 0.0000 . 14:0 . . 16.800 . .|. 29030 ..| .29030 . . ..0 . . . . . . . . 0
15.16 . . 1.0825 . 14:1 . . 18.186 . .|. 31462 ..| .31425 . . .36 . . . . . . . 1.00
15.16 . . 1.2629 . 12:1 . . 18.186 . .|. 31468 ..| .31425 . . .42 . . . . . . . 1.17
15.16 . . 1.5155 . 10:1 . . 18.186 . .|. 31476 ..| .31425 . . .51 . . . . . . . 1.40
*Assuming the volume displaced by one liquid oz gasoline = 1.80468 (cu-in) and a Vapor Liquid ratio of 20.
Now keeping in mind that engine is essentially an air pump - we can see the effect of running these volumes or 'helpings' through an engine with a particular displacement.
Take for example my personal favorite:
Engine. . . . . . . . . . . Per Rev
1949 216.5 (cu-in) . . . . 54.12500
. . . .. . v/e 0.85 . . . . . 46.00625
Now Let's assume that we consume these three theoretical 'helplings' of fuel/air a total of 384 times each over the course of an hour:
Ratio . . Volume (cu-in) . . Revolutions . .RPH . . . . .RPM . . Fuel oz . . . . Gallons
14:1 . . . . 31462 . . . . . . 683.85 . . . 262599 . . . . 4376.66 . . 384 . . . . . . 3
12:1 . . . . 31468 . . . . . . 683.98 . . . 262650 . . . . 4377.49 . . 448 . . . . . . 3.5
10:1 . . . . 31476 . . . . . . 684.17 . . . 262720 . . . . 4378.67 . . 538 . . . . . . 4.2
Again we're looking at these numbers from a common sense perspective - so the question is - do these numbers pass the 'smell' test?
Consider the RPM required to consume 384 helpings - how fast does it imply the drive train is carrying the vehicle?
In the case of a '49 chevy fastback with a 4.11 rear-end and 26 inch rubber we'd be traveling in excess of 80 miles per hour when cruising at this RPM.
And now that we have added a distance dimension to our 'volume' model we can calculate efficiencies:
Ratio . . . .MPG @80 @4377 RPM
14:1 . . . . 26.67 (3.0 gals)
12:1 . . . . 22.86 (3.5 gals)
10:1 . . . . 19.05 (4.2 gals)
What happens if we change the rear gear to a 3.55? But we hold the RPM constant:
Ratio . . . .MPG @92
14:1 . . . . 30.83
12:1 . . . . 26.42
10:1 . . . . 22.02
What happens if we add a .72 Overdrive on-top of the 3.55 gear? But again holding the RPM constant:
Ratio . . . .MPG @128
14:1 . . . . 42.67
12:1 . . . . 36.57
10:1 . . . . 30.48
If at this point you are thinking 'balderdash' - then we're on the same page!
Let's instead add the 3.55 gear while holding the speedo constant. RPM drops by (3.55/4.11) as does the volume of the fuel/air 'helpings' consumed:
Ratio . . . .MPG @80 @3780 RPM
14:1 . . . . 30.87 (2.6 gals)
12:1 . . . . 26.46 (3.0 gals)
10:1 . . . . 22.05 (3.6 gals)
And then add a .72 over drive again while holding the speedo constant. RPM drops by (3.55/4.11)*.72 as does the volume of the fuel/air 'helpings' consumed:
Ratio . . . .MPG @80 @2722 RPM
14:1 . . . . 42.88 (1.9 gals)
12:1 . . . . 36.75 (2.2 gals)
10:1 . . . . 30.63 (2.6 gals)
What is interesting to me is the symmetry that results from such single dimensional analysis. A 10:1 ratio can be used either to go 80 MPH @2722 RPM or 128 MPH @4377 at the exact same efficiency 30.5 MPG?
But this too perhaps belies the smell test when other dimensions are considered.
What about the power contained in a gallon of gasoline? The theoretical number is 49HP/hour - but physics says about 65% of that is lost to heat in the process - leaving perhaps 17 HP/hour per gallon.
Is 71 HP enough to maintain 128 MPH? (17 X 4.2 gallons)
Is 44 HP enough to maintain 80 MPH? (17 x 2.6 gallons)
When viewed from this perspective the ability of an OD to extend efficiency at lower RPMs seems more likely then it's ability to extend top end speed at a given RPM without actually burning more fuel. The former is about efficiency while the latter is about power (which an engine is either making or it isn't). Moreover, in the same way that on OD doesn't make power - it doesn't rob it either at cruise.
I am including some links that I have found useful:
Calculating Wheel HP It is surprising how few HP are really required to maintain speed at cruise - and that wind resistance is at play - not weight.
The impact of small things on an engine at cruise is equally interesting:
Day Time Running Lights and MPG head lights, pumping losses? . . . so many dimensions to get one's head around when pondering efficiency at cruise.
See where I am coming from?
regards,
stock49